This proposal is to replace an existing, outmoded, X-ray crystallographic area detector with a newer instrument. The older instrument, an Enraf Nonius FAST area detector mounted on a FR571 rotating anode X-ray generator, was and continues to be heavily used, and has yielded eight productive years of diffraction data. The older instrument, however, lacks many of the advantages found in the latest devices and is increasingly subject to failure. None of the components in the older instrument are currently manufactured, and, soon, none will be supported by the manufacturer. We are therefore requesting a complete new system. The proposed instrument will reside in the PI's laboratory, as does the older instrument. Since the PI's laboratory represents the only macromolecular crystallography group at the University of Arizona, oversight of the instrument and training of individuals in the use of the instrument will be performed by the PI and his group. Macromolecular crystallography is of central importance to biology, medicine, and rational drug design. The PI and Co-PI's are studying a variety of biological topics, including nitric oxide transport by blood- sucking insects, several additional antihemostatic proteins, two anti- cancer drug targets, and peptidomimetic hormones and neurotransmitters. These studies are supported by four NIH grants and several non-NIH grants, and the structural portions of these studies depend on the PI's X- ray facility. For example, structural collaborations between the PI and the Co-PIs have resulted in seven publications in the past two years. Continued work in these on-going studies will soon be impossible without upgrading our X-ray facility.